Here's a follow-up. How does a bare buffer tube get by without being designated a stock? Because often times these buffer tubes can be long enough to funtion as a stock.
Also, I've seen people put elaborate "covers" on their buffer tubes. When does a cover become a fixed stock?
ATF defines a stock as something designed to be mounted to and fired from the shoulder. The buffer tube or receiver extension is not manufactured with the intent of being mounted to the shoulder, but rather it is intended to house the buffer, spring, and reciprocating bolt carrier group. So it is by definition not a stock regardless of how people use it.